1. Field of the Invention
The present invention relates generally to nuclear reactor coolant system pumps and, more particularly, is concerned with an assembly for facilitating inservice inspection of a reactor coolant pump rotor.
2. Description of the Prior Art
Improvements in safety, power generation cost, reliability and maintainability are the goals of a new generation of pressurized water nuclear reactors. An innovative reactor coolant system planned for use in the new generation PWR provides improvements in all of these areas while employing proven components in a unique compact configuration. One component employed by the reactor coolant system is a canned motor pump. Two canned motor pumps are used in each of the two coolant loops of the reactor coolant system. The pump casing of the canned motor pump is attached directly to the bottom of the channel head of the steam generator in the reactor coolant system. This configuration effectively combines the two components into a single structure.
The advantages of this configuration are significant. The canned motor pump has a demonstrated record of high reliability and an inherently simplified auxiliary fluid system. This pump further diminishes the potential risk for small loss-of-coolant accident (LOCA) because shaft seals are not used. Further, the close coupling of the pump and the steam generator eliminates a piping leg, improves loop pressure drop, simplifies the support system, and reduces potential for core uncovery during small LOCA.
A preventive maintenance program has been established for the canned motor pumps. This program should be performed between major inspections to verify the integrity of the pumps, prevent premature failure of the component and minimize plant down time. A mechanical inspection, scheduled to be performed at each refueling, consists of tests of breakaway torque (to confirm that the pump rotor spins or rotates freely and no rubs exist) and axial end play (check for excessive thrust bearing wear).
However, the hermetically-sealed canned motor pump is mounted to the bottom of the steam generator channel head in an inverted position where the canned motor stator flange of the pump is located well below the reactor vessel flange. Due to this arrangement, the water level in the refueling canal during refueling operations is approximately 45 feet above the bottom of the canned motor pump, making it impossible to physically reach the pump rotor through an inspection port without draining the pump and the system.
It is highly desirable to perform the aforementioned mechanical inspection without draining the system, without disassembling the pumps and without interfering with the critical path. Also, the breakaway torque test can be performed with less possibility of damage if the thrust and radial bearing are water lubricated rather than in the dry condition. Further, to minimize the impact of the canned motor pump mechanical inspection on plant availability, it is desirable that the inspection be performed during plant schedule outage for refueling notwithstanding the considerable distance of the pump below water level.
Consequently, there is a need for an approach to inservice pump inspection which permits performance of the pump rotor inspection during plant refueling and while the pump is submerged deep in water.